JOURNAL OF RARE EARTHS, Vol. 32, No. 1, Jan. 2014, P. 90 Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite-hosted heavy rare earth element (HREE) deposit PINGITORE Nicholas1,*, CLAGUE Juan1, GORSKI Daniel2 (1. Department of Geological Sciences, The University of Texas at El Paso, El Paso, TX 79968-0555, USA; 2. Texas Rare Earth Resources, 539 El Paso St., Si- erra Blanca, TX 79851, USA) Received 16 September 2013; revised 9 November 2013 Abstract: Round Top Mountain in Hudspeth County, west Texas, USA is a surface-exposed rhyolite intrusion enriched in Y and heavy rare earth elements (HREEs), as well as Nb, Ta, Be, Li, F, Sn, Rb, Th, and U. The massive tonnage, estimated at well over 1 billion tons, of the deposit makes it a target for recovery of valuable yttrium and HREEs (YHREEs), and possibly other scarce ele- ments. Because of the extremely fine grain size of the mineralized rhyolite matrix, it has not been clear which minerals host the YHREEs and in what proportions. REE-bearing minerals reported in the deposit included bastnäsite-Ce, Y-bearing fluorite, xeno- time-Y, zircon, aeschynite-Ce, a Ca-Th-Pb fluoride, and possibly ancylite-La and cerianite-Ce. Extended X-ray absorption fine struc- ture (EXAFS) indicated that virtually all of the yttrium, a proxy for the HREEs, resided in a coordination in the fluorite-type crystal structure, rather than those in the structures of bastnäsite-Ce and xenotime-Y. The YREE grade of the Round Top deposit was just over 0.05%, with 72% of this consisting of YHREEs. This grade was in the range of the South China ionic clay deposits that supply essentially all of the world’s YHREEs. Because the host Y-bearing fluorite is soluble in dilute sulfuric acid at room temperature, a heap leaching of the deposit appeared feasible, aided by the fact that 90%–95% of the rock consists of unreactive and insoluble feld- spars and quartz. The absence of overburden, remarkable consistency of mineralization grade throughout the massive rhyolite, prox- imity (a few km) to a US interstate highway, major rail systems and gas and electricity, temperate climate, and stable political location in the world’s largest economy all enhanced the potential economic appeal of Round Top. Keywords: heavy rare earth elements; yttrium-bearing fluorite; yttrofluorite; heap leaching; rhyolite; Round Top; deposit The rare earth elements (REEs) are essential compo- neous rock body related to granite, emplaced as a mush- nents of current and emerging 21st century technologies. room-shaped mass between sedimentary rock layers), en- Recent concern about future supplies of all the REEs riched in rare earths elements, particularly the scarce now has narrowed chiefly to the heavy rare earth ele- YHREEs: Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. The rhyolite ments (HREEs). Essentially all the world’s HREEs cur- is also enriched in Be, Ga, Sn, Nb, Ta, U, and Th. The de- rently are sourced from the south China ion-adsorption posit lies some 130 km southeast of El Paso, TX (Fig. 1[1]). clays. The ability of those deposits to maintain and in- Development of a beryllium mine at Round Top in the crease production is uncertain, particularly in light of en- 1980s was later abandoned. The beryllium deposit lies vironmental degradation associated with some mining along part of the lower contact with limestone; the target and extraction operations in the region. was 3×105 tons of 2% BeO mineralization[1]. The prop- We investigated an yttrium and HREE (YHREE) de- erty now is leased from the General Land Office of the posit that has proven to be unique in its mineralogy and State of Texas by Texas Rare Earth Resources, a US impressive in its tonnage. This unconventional deposit corporation. TRER plans to recover the REEs; it is presents the potential to become a significant source for evaluating simultaneous extraction of lithium, uranium, [1] YHREEs in the near future. and beryllium from the rhyolite as well . The Round Top Mountain rhyolite is enormous; the Texas Bureau of Economic Geology estimated the rock 1 Round Top Mountain HREE deposit mass at 1.6 billion tons[2]. The mountain itself is ap- proximately 375 m tall and nearly 2 km in diameter. 1.1 Nature and characteristics of the deposit Measured, indicated, and inferred total REEs (TREEs) Round Top Mountain, at Sierra Blanca, west Texas, exceed 5×105 t[3]. Approximately 72% of the TREEs in USA, is a rhyolite laccolith intrusion (a fine-grained ig- the rhyolite are the scarce YHREEs[3]. Foundation item: Project supported by Research Contracts 26-8211-12 and 26-8211-16 between Texas Rare Earth Resources, Inc. and the University of Texas at El Paso * Corresponding author: PINGITORE Nicholas (E-mail: [email protected]; Tel.: +1-915-747-5754) DOI: 10.1016/S1002-0721(14)60037-5 PINGITORE Nicholas et al., Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite- ... 91 Fig. 1 Location of Round Top Mountain deposit The deposit is exposed as an erosional remnant that containing grains that could be easily beneficiated by can easily be surface-mined, with essentially no cover to such physical means as gravity or magnetic separation be removed (Fig. 2[1]). Although huge in extent, well ex- are scarce. The REE-bearing minerals are not concen- posed, easily accessible by rail and US interstate high- trated in veins within the rhyolite that could be extracted way, and proximal to electricity and natural gas, the min- by selective mining. eralization is both dilute (TREE grade just over 500 ppm) We have been experimenting in the laboratory at and diffuse (no concentrated zones)[2,3]. The ability to bench scale with direct acid leaching to assess heap economically tap this potential world-class resource leaching as a possible route to low-cost YHREE extrac- hinges on extraction from an ore grade an order of mag- tion. Previous studies indicated that very fine grinding nitude (one-tenth) or more lower than those in conven- and froth floatation concentrates the target YHREEs, but tional REE mining operations, which typically are in the this approach requires a large initial capital expenditure % range. It is, however, within the range of the HREE and significant operating complexity and expenses[3]. ion-adsorption clays of southern China[4]. Further, in ad- Successful extraction of the YHREEs at Round Top by dition to their homogeneous distribution (very little heap leach would be a breakthrough in REE mining variation in concentration throughout the rhyolite), the technology. REEs are hosted in a matrix that is extremely fine grained (generally sub-micron in diameter). Potassium 1.2 REE mineralogy at Round Top feldspar, plagioclase, and quartz grains comprise the bulk, The mineralogy of the YREEs and other critical ele- 90%–95% of the volume, of the rhyolite. Large REE- ments is essential to develop an appropriate extraction Fig. 2 Round Top Mountain, virtually all of which is mineralized rhyolite 92 JOURNAL OF RARE EARTHS, Vol. 32, No. 1, Jan. 2014 protocol. Previous study of the elemental composition of Table 1 HREE and Y average concentrations in Round Top mineral grains by electron microprobe indicates that the Mountain rhyolite REEs are hosted by bastnäsite-Ce, Ce-bearing fluorite, Elements Eu Gd Tb Dy Ho Er Tm Yb LuY Y-bearing fluorite, xenotime-Y, zircon, aeschynite-Ce, a Concentration/(g/t) 0.2 10.6 3.6 31.7 8.0 32.8 7.1 56.5 8.9 221 Ca-Th-Pb fluoride, and possibly ancylite-La and ceri- Note: Data from Ref. [3] anite-Ce[2,3,5,6]. It was suspected, but never demonstrated, that most of the REEs were hosted in various fluorine minerals. The relevant microprobe analyses were limited 2 Materials and methods by grain size: only “large” particles, mostly 5–10 µm in dimension, could be examined. Aside from the feldspar 2.1 Rhyolite samples and quartz phenocrysts, most of the remaining 5%–10% All but one sample were split from a well-mixed >250 of the rock volume comprises grains a micrometer or less kg sample composited from material recovered from a in diameter. These are not suitable for electron probe mi- representative set of reverse circulation exploration drill croanalysis. The fraction of the REEs in each mineral, or holes into the Round Top Mountain rhyolite. This mate- in unidentified phases too small to analyze, was not, and rial assayed at 211 g/t yttrium. Table 2 lists two samples probably could not be estimated. Optical or microprobe that were sieved to the specified grain size, and a third point counting of grains is the conventional way to quan- sample that was a small block cut from a large grain re- tify the fraction of an element in different phases. This is covered from the drill debris. The remaining 5 samples not possible due to both the low concentration (for Yb at were size-sorted granular rhyolite that was leached for 50 ppm, one searches through a million grains in an at- various periods and temperatures in 8.3 vol.% sulfuric tempt to encounter a single grain of fluorite that is, say, acid. The amount of the initial yttrium remaining in the 2% ytterbium) and minuscule grain size (analogous to samples, relative to the respective untreated material “invisible” gold) of the critical elements in the bulk rock. (first 2 samples), is listed. All samples, except for the Lacking firm knowledge of the mineralogy of the REEs, block, were subsequently reduced in a ball mill to <10 development of an extraction process is forced to be μm grain size, and pressed into a standard 25-mm X-ray based on an extensive and expensive program of trial and fluorescence puck with the addition of a cellulose binder error.